1. Field of the Invention
This invention relates to a plug cap device to be applied to a DOHC (double overhead cam shaft) type gasoline engine or the like.
2. Statement of the Prior Art
For convenience of explanation, a prior plug cap device will be explained below. FIG. 27 is a side view of a prior plug cap device, in which a main part of the device is shown in a cross sectional view. As shown in FIG. 27, the prior plug cap device comprises an elongate cylindrical plug cap body 10 made of a synthetic resin and a rain cover 20 which constitutes a water-proof cover. The plug cap body 10 is provided with a rib 11 on the outer periphery of an upper end. A cylindrical rubber cap 12 is mounted on a lower end of the body 10.
The rain cover 20 made of an elastic material is fitted on the upper end of the plug cap body 10 with the rib 11 on the body 10 being mated with an annular recess 22 formed on the inner periphery of an aperture 21 in the cover 20. A vent hole 23 is formed in an upper portion of a wall defining the aperture 21 in the cover 20. A groove 25 is formed ill the inner periphery of the aperture 21 at positions from the vent hole 23 to the lower end of the rain cover 20. An air flow passage 24 is defined between the inner periphery of the groove 25 and the outer periphery of the plug cap body 10.
An ignition cable 30 is inserted from the upper end of the rain cover 20 through a body of the cover 20 into the plug cap body 10. A discharge tube unit and a connecting terminal (not shown) connected to an end of the ignition cable 30 is received in a lower half portion of the plug cap body 10.
The plug cap body 10 is inserted into a plug bore 2 in an engine body 1 and the connecting terminal received in the lower end of the plug cap body 10 is coupled together with a rubber cap 12 to a plug secured to an attaching portion 3 on the bottom of the plug bore 2. Further, a lower portion 20a of the rain cover 20 is airtightly engaged with an upper end of the plug bore 2, so that the rain cover 20 seals the plug bore 2 from an external area except the vent hole.
In the plug cap device, when an inner pressure rises due to a rise in temperature, air trapped in the plug bore 2 is discharged through the air flow passage 24 from the vent hole 23 into tile external area, thereby avoiding a rise of an inner pressure In the plug bore 2 and avoiding of floating the rain cover 20 due to the rise of the inner pressure in the bore 2.
In the plug cap device described above, since some elements such as the discharge tube unit and the like are received in the plug cap body 10, an outer diameter of the body 10 becomes large and thus an inner diameter of the aperture 21 in the rain cover 20 becomes large. On the contrary, since a diameter of the plug bore 2 is maintained to be constant, a thickness of the rain cover 20 must be thin. If the groove 25 for the air flow passage is formed in the rain cover 20, the thickness of the cover 20 at the groove 25 becomes thinner and this causes a deterioration in rigidity of the rain cover 20.
When an engine or a body in a motor car is washed, water enters from the vent hole 23 through the air flow passage 24 into the plug bore 2 in the prior plug cap device. The water in the plug bore 2 causes a deterioration in electrical insulation of the plug cap body 10 and thus causes an ignition voltage at the ignition plug to be lowered and the ignition plug to be fired.